New test developed at CVM to detect Brucella canis
Researchers at the Cornell University College of Veterinary Medicine (CVM) have developed a new test to diagnose Brucella canis, a zoonotic disease that can spread to people through contact with infected dogs.
The Cornell Animal Health Diagnostic Center (AHDC) is the only lab in the country offering a new test that measures antibodies in dogs directed against Brucella canis antigens — the Canine Brucella Multiplex (CBM) assay. Shortly before it was released, a popular in-clinic assay for the diagnosis of the disease was discontinued, resulting in a doubling of the number of dogs being tested each month at the AHDC.
“We’re testing hundreds of dogs per month,” said Dr. Toby Pinn-Woodcock, assistant clinical professor in Veterinary Support Services at CVM. “We believe that this test is more sensitive and less prone to false-positive results.”
The disease, which affects up to six percent of dogs in the United States, shows a variety of symptoms, from spontaneous abortion to spinal pain, and therefore is difficult to detect. The average time to identify Brucella canis in a dog is about six months according to a paper coauthored by Pinn-Woodcock and five other Cornell researchers, which was published in the American Journal of Veterinary Research in March.
The lengthy diagnosis has raised concerns among veterinarians and state health officials because Brucella canis can spread to humans, causing flu-like symptoms, organ-based complications involving the skeletal system, and possible problems with pregnancy.
“We always worry about the potential that a pregnant woman could have pregnancy complications associated with this,” said Dr. Alexandra Newman, the state public health veterinarian with the New York State Department of Health. “We also worry about meningitis and swollen lymph nodes, and Brucella getting into joints and causing arthritis.”
The disease, however, has been rarely diagnosed in people in New York State. In the past 16 years, Newman said there is only one known case of Brucella canis being transmitted from a dog to a person. That case involved a puppy purchased from a commercial breeder that spread the disease to its owner.
People who are most at risk for infection are owners of dogs that have the disease, workers at commercial breeding operations, or laboratory technicians who culture the organism. The disease is more prevalent among rescued stray dogs or dogs raised at large breeding operations.
Despite the low transmission rate, Newman said she believes that the disease is underdiagnosed. “People don’t suspect it,” she said. “The disease isn’t always severe, and it doesn’t land people in the hospital, and it’s also hard to diagnose.”
Using the new assay to monitor recovery
Dogs diagnosed with Brucella canis are treated with a combination of antibiotics, often long-term. If treatment is effective, the levels of antibodies directed against the organism are expected to decrease.
Yet it is difficult to determine when a dog has been completely cleared of the infection because the pathogen can evade immune detection. The challenges in curing the disease have led many veterinarians who diagnose dogs with Brucella canis to recommend that they be euthanized.
“It can be very hard to completely eliminate from animals because the pathogen essentially lives within the cells,” Newman said. “The only way we can guarantee that there will be no risk of human exposure is by euthanizing the dog.”
In New York State, pet owners are allowed to decide whether to euthanize or treat the animal. The first recommendation is to neuter the dog. “Preventing the breeding of infected dogs decreases the likelihood that they will spread the infection,” said Cassandra Guarino, an assistant professor of practice in the Population Medicine and Diagnostic Sciences Department at CVM.
With the new CBM assay, veterinarians can now monitor how well dogs respond to treatment for the disease. Unlike previous tests offered at the AHDC for Brucella canis, the new test is quantitative and can detect the level of antibodies in the serum of an infected dog, not just whether the antibodies are present or not.
In their recent study, the Cornell researchers found that a decrease in antibody values directed against one of the two antigens in the CBM assay was associated with a resolution of the animal’s clinical signs. That decline in antibodies, however, did not mean that the infection was no longer present because it could still be sequestered in the animal’s tissue.
“Brucella canis is so well-adapted to the canine host that it is able to persist long-term in that host,” Guarino said. “In many cases, the organism can go into a dormant state within the canine host. At some point, prompted by a stress event or other immune compromise, the bacteria can reemerge.”
Guarino said one infected dog described in the study showed a 40 percent drop in antibody levels, but when it was retested three months later, the antibody levels had risen, indicating that the animal was still infected.
The research team is seeking funding to conduct a future study that would show whether dogs that have a prolonged drop in antibody levels are cured of the disease. “In theory, if the [antibody] values come down and stay down for a certain amount of time, we may be able to say with confidence that the animal has successfully cleared the infection, but we’re not quite there yet,” Guarino said.
Until further studies are done, Pinn-Woodcock said she wants veterinarians across the country to know that the new test may provide a means of monitoring response to antibiotic treatment in dogs diagnosed with Brucella canis.
Written by Sherrie Negrea